A battery has widely been used as a power source of a portable electronic device, such as a game machine and a portable music player. The operation of such a portable electronic device stops when the residual quantity of the battery thereof has decreased. Accordingly, the user thereof sometimes continues the operation thereof by connecting an AC adaptor without exchanging the battery. In such a case, it is desirable for internal circuits from the point of view of their stability to perform their operations by the use of the DC voltage from the AC adaptor in place of the power source voltage from the battery.
Accordingly, a power source switching apparatus having the function of selecting the DC voltage of any one of a plurality of DC power sources to supply the selected voltage to a load has been provided in the past. A transistor, such as a metal oxide semiconductor (MOS) field-effect transistor (FET) (or insulated-gate field-effect transistor; hereinafter referred to as MOS transistor), is used for a switching switch in such a power source switching apparatus.
There are P channel type MOS FET and N channel type MOS FET. If an N channel MOS FET is used as a switching element for the transmission/interception of a DC voltage in a power source switching apparatus, such a switching element has a problem of the enlargement of the circuit size thereof owing to a charge pump, which becomes necessary to be provided because the channel of the switching MOS FET conducts in a state in which the potential of the source thereof and the drain thereof is nearly a power source voltage in the on-state thereof and the on-resistance of the switching MOS FET becomes large to cause much loss unless the gate voltage is set to be higher than the power source voltage.
On the other hand, if a P channel MOS FET is used as a switching element for the transmission/interception of a DC voltage, no charge pumps are necessary because the channel can conduct by applying the ground potential to the gate terminal. But, if the P channel MOS FET and the N channel MOS FET have the same size, the P channel MOS FET has a larger on-resistance than that of the N channel MOS FET because the P channel MOS FET has holes as the carriers thereof and the N channel MOS FET has electrons as the carriers thereof. Accordingly, the P channel MOS FET having a large size must be used, and consequently the occupation area of the switching element is large. Because the increased quantity of the chip size owing to the large occupation area is larger than the decreased quantity of the chip size owing to the omission of the charge pump, the P channel MOS FET has a problem of the increase of the chip size thereof also in the case of total consideration.
Moreover, even if either of the P channel type and the N channel type MOS FETs is used as the switching element, a device for preventing a reverse current from flowing through a parasitic diode existing between the source and drain region of the switching element and the substrate thereof during the off-state thereof becomes necessary. The inventions, each preventing a reverse current by applying the higher one of an input voltage and an output voltage to a substrate (well region) as a substrate bias voltage in a power source apparatus (regulator) using a P channel MOS FET as a switching element for transmission/interception, have been proposed in the past (see Patent Literature 1 and Patent Literature 2).